This invention relates to boron isotope enrichment, and, more particularly, to a laser-induced photochemical method for the selective enrichment of either boron-10 or boron-11 isotopes.
There has been much recent interest in the development of various methods of isotope separation and enrichment. Although efforts along these lines have primarily been directed to the separation of uranium isotopes for nuclear fuels in the hope of achieving a method more economical than the very expensive ones currently used, pure or enriched isotopes and isotopic compounds of the lighter elements, such as boron and chlorine, are also much needed, for example, as tracer materials for medical research and diagnosis, biological research, and environmental studies.
A number of previously proposed isotope separation and enrichment methods have utilized laser radiation for selectively exciting particular isotopes or isotope-containing molecules. The selectively excited isotopic species must then be removed before it has a change to decay back to the ground state or be involved in energy transfer collisions with other isotopic species. Such laser-initiated procedures up to now have been, for the most part, photophysical in nature, wherein one- or multi-photon processes photodissociate molecules or photoionize or deflect atoms, all being unimolecular processes. The principal disadvantage of these photophysical processes has been their inherent overall low efficiency. Although attempts have beem made to improve the efficiency of laser-induced isotope separation and enrichment through the use of photochemical techniques whereby the selectively excited isotopic species is removed by chemical reaction with a reactant added to the system, such attempts heretofore have not resulted in stable reaction products being formed and have failed to achieve appreciable yields of isotopically enriched materials.
Photochemical isotope enrichment techniques are based on two main phenomena. First, there is the fact that the wave lengths of spectral lines absorbed by a molecule depend somewhat on the isotopes present in the molecule. Second, the rate of a chemical reaction is sometimes influenced by the state of excitation of the participating molecules. Although the precise mechanisms of the latter process are not very well understood, educated guesses can occasionally be made as to which excitations are likely to accelerate a given reaction. In order for photochemical isotope enrichment to be possible with a given starting material, several conditions must be satisfied. First of all, the effect of isotopic content of the starting material on the wave lengths of one or more of its spectral lines must be large enough so that one type of isotope-containing molecule could be preferentially excited by absorbing laser radiation which would not excite the other types of isotope-containing molecules. Secondly, a laser is needed whose radiation happens to match in wave length one of the isotope-dependent lines, or a laser that can be tuned to such a wave length, and the spectral width of the laser radiation must be narrow enough to excite molecules containing one of the isotopes and not the others. Thirdly, the isotope-containing starting compound must be capable of being mixed with other substances with which it is known to react fairly slowly, but which can be made to react more rapidly when one of the isotopic species is selectively excited by the radiation chosen. Fourthly, transfer of excitation from one molecule to another by collision, and "scrambling" of isotopes through collision of reaction products with other reactive species (for example, free radicals) present, must be negligible, since both of these factors tend to reduce the selectivity of the overall process.